In recent years, an organic electroluminescent element (hereinafter, also referred to as an organic EL element) has been extensively developed. An organic EL element is a thin film element comprising at least one layer of an organic layer (including an emission layer) being sandwiched by electrode thin films, and applications thereof for various displays, which are capable of obtaining emission colors from red to blue by forming the organic layer with (appropriate) selection among fluorescent substances of various organic low molecular weigh compounds and polymer compounds, have been studied.
A bottom emission type organic EL element is constituted of, for example, transparent conductive film such as ITO film having being patterned as a bottom electrode on a transparent substrate, an organic layer containing an emission layer in addition to cathode as a reflective electrode which are accumulated on said anode, and emission is taken out from the substrate side.
FIG. 1 is a drawing to show an example of a bottom emission type organic EL element of a simple matrix type. In a simple matrix type, ITO (indium tin oxide) film is patterned into a stripe form to form transparent electrode (anode) 2, organic layer 3 having been further provided, and then cathode 4 (also referred to as a reflective electrode) comprising such as aluminum is formed also in a stripe form so as to cross straight against the transparent electrode pattern (herein, this cathode layer is abbreviated in FIG. 1 (a) because it is formed covering the whole surface).
In these elements, an overlapping portion of an anode and a cathode forms an emission area, however, it has been proved that there caused a phenomenon to lose emission in the edge portion of a cathode in an emission area when light-on or current-on is continued for a long period resulting in reduction of the light emission area (the region is 15 in FIGS. 1 (a) and (b)). This is considered to be related to a cathode (a reflective electrode) being present on the outside (the atmospheric side).
This invention provides a bottom emission type organic EL panel in which arrangement of electrodes constituting said panel is designed to prevent or restrain reduction of the aforesaid emission area. Herein, it is disclosed that improvement is intended by conducting sealing of an organic EL by a cathode having an increased film thickness (for example, refer to patent document 1). However, in this document, it is sealing of an active matrix organic EL panel, which is a relationship between organic film and cathode film, and is different from relationship between an emission area and a cathode area according to this invention. An only specific means disclosed to avoid contact of a cathode and a wiring, which is caused by forming a cathode having a larger size than organic film, is a means by a sealing material. Further, since a cathode layer thickness is increased, it is expected to cause a problem of such as stress in an emitting substance having a relatively large area in this invention.
Further, it is known an example of a segment emission organic EL element of a bottom emission type in which thin film comprising a metal, which is provided with a work function of not more than 4.8 eV such as aluminum, is arranged around an ITO electrode in a similar manner to this invention (for example, refer to patent document 2), however, the purpose is different from this invention.
Further, disclosed is a constitution to make an emission area to be smaller than cathode area by use of such as an insulation film or a supplemental electrode (for example, refer to patent documents 3 and 4). However, this invention proposes a method not only to make an emission area smaller than a cathode area but also to make a primary emission area smaller than a cathode area to form a constitution, in which a low emission area and a cathode area are brought in contact to reduce an effect of loss. Simultaneously, a means to make a main emission area smaller than cathode area is also different.
Patent Document 1: JP-A 2003-178885 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection)
Patent Document 2: JP-A 2000-357589
Patent Document 3: JP-A 2001-267066
Patent Document 4: JP-A 2000-21564